Network Working Group D. Ooms, B. Sales
Internet Draft Alcatel
Expiration Date: JuneJuly 2001 W. Livens
Colt Telecom
A. Acharya
IBM
F. Griffoul
Ulticom
F. Ansari
Bell Labs
December 2000January 2001
Framework for IP Multicast in MPLS
draft-ietf-mpls-multicast-04.txtdraft-ietf-mpls-multicast-05.txt
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Abstract
This document offers a framework for IP multicast deployment in an
MPLS environment. Issues arising when MPLS techniques are applied to
IP multicast are overviewed. The pros and cons of existing IP
multicast routing protocols in the context of MPLS are described and
the relation to the different trigger methods and label distribution
modes are discussed. The consequences of various layer 2 (L2)
technologies are listed. Both point-to-point and multi-access
networks are considered.
Table of Contents
1. Introduction
2. Layer 2 characteristics
3. Taxonomy of IP multicast routing protocols in the context of MPLS
3.1. Aggregation
3.2. Flood & Prune
3.3. Source/Shared trees
3.4. Co-existence of Source and Shared Trees
3.5. Uni/Bi-directional Shared Trees
3.6. Encapsulated multicast data
3.7. Loop-free-ness
3.8. RPF Check
3.9. Mapping of characteristics on existing protocols
4. Mixed L2/L3 forwarding in a single node
5. Taxonomy of IP multicast LSP triggers
5.1. Request driven
5.1.1. General
5.1.2. Multicast routing messages
5.1.3. Resource reservation messages
5.2. Topology driven
5.3. Traffic driven
5.3.1. General
5.3.2. An implementation example
5.4. Combinations of triggers and label distribution modes
6. Piggy-backing
7. Explicit routing
8. QoS/CoS
8.1 DiffServ
8.2 IntServ and RSVP
9. Multi-access networks
10. More issues
10.1. TTL field
10.2. Independent vs. Ordered Label Distribution Control
10.3. Conservative vs. Liberal Label Retention Mode
10.4. Downstream vs. Upstream Label Allocation
10.5. Explicit vs. Implicit Label Distribution
11. Security Considerations
12. Acknowledgements
Table of Abbreviations
ATM Asynchronous Transfer Node
CBT Core Based Tree
CoS Class of Service
DLCI Data Link Connection Identifier
DRrecv Designated Router of the receiver
DRsend Designated Router of the sender
DVMRP Distant Vector Multicast Routing Protocol
FR Frame Relay
IGMP Internet Group Management Protocol
IP Internet Protocol
L2 layer 2 (e.g. ATM, Frame Relay)
L3 layer 3 (e.g. IP)
LSP Label Switched Path
LSR Label Switching Router
LSRd Downstream LSR
LSRu Upstream LSR
MOSPF Multicast OSPF
mp2mp multipoint-to-multipoint
p2mp point-to-multipoint
PIM-DM Protocol Independent Multicast-Dense Mode
PIM-SM Protocol Independent Multicast-Sparse Mode
QoS Quality of Service
RP Rendezvous Point
RPF Reverse Path ForwardingRPT-bit RP Tree bit [DEER]
RSVP Resource reSerVation Protocol
SPT-bit Shortest Path Tree [DEER]
SSM Source Specific Multicast
TCP Transmission Control Protocol
UDP User Datagram Protocol
VC Virtual Circuit
VCI Virtual Circuit Identifier
VP Virtual Path
VPI Virtual Path Identifier
Changes - clean up references - changes to section of Explicit
Routing - added SSM to multicast protocol taxonomy
1. Introduction
In an MPLS cloud the routes are determined by a L3 routing protocol.
These routes can then be mapped onto L2 paths to enhance network
performance. Besides this, MPLS offers a vehicle for enhanced
network services such as QoS/CoS, traffic engineering, etc.
Current unicast routing protocols generate a same (optimal) shortest
path in steady state for a certain (source, destination)-pair. Remark
that unicast protocols can behave slightly different with regard to
equal cost paths.
For multicast, the optimal solution (minimum cost to interconnect N
nodes) would impose a Steiner tree computation. Unfortunately, no
multicast routing protocol today is able to maintain such an optimal
tree. Different multicast protocols will therefore, in general,
generate different trees.
The discussion is focused on intra-domain multicast routing
protocols. Aspects of inter-domain routing are beyond the scope of
this document.
2. Layer 2 characteristics
Although MPLS is multiprotocol both at L3 and at L2, in practice IP
is the only considered L3 protocol. MPLS can run on top of several
L2 technologies (PPP/Sonet, Ethernet, ATM, FR, ...).
When label switching is mapped on L2 switching capabilities (e.g.
VPI/VCI is used as label), attention is mainly focused on the mapping
to ATM [DAVI]. ATM offers high switching capacities and QoS
awareness, but in the context of MPLS it poses several limitations
which are described in [DAVI]. Similar considerations are made for
Frame Relay on L2 in [CONT]. The limitations can be summarized as:
- Limited Label Space: either the standardized or the implemented
number of bits available for a label can be small (e.g. VPI/VCI
space, DLCI space), limiting the number of LSPs that can be
established.
- Merging: some L2 technologies or implementations of these
technologies do not support multipoint-to-point and/or multipoint-
to-multipoint 'connections', obstructing the merging of LSPs.
- TTL: L2 technologies do not support a 'TTL-decrement' function.
All three limitations can impact the implementation of multicast in
MPLS as will be described in this document.
When native MPLS is deployed the above limitations vanish. Moreover
on PPP and Ethernet links the same label can be used at the same time
for a unicast and a multicast LSP because different EtherTypes for
MPLS unicast and multicast are defined [ROSE].
3. Taxonomy of IP multicast routing protocols in the context of MPLS
At the moment, an abundance of IP multicast routing protocols is
being proposed and developed. All these protocols have different
characteristics (scalability, computational complexity, latency,
control message overhead, tree type, etc...). It is not the purpose
of this document to give a complete taxonomy of IP multicast routing
protocols, only their characteristics relevant to the MPLS technology
will be addressed.
Following characteristics are considered:
- Aggregation
- Flood & Prune
- Source/Shared trees
- Co-existence of Source and Shared Trees
- Uni/Bi-directional shared trees
- Encapsulated multicast data
- Loop-free-ness
- RPF check
The discussion of these characteristics will not lead to the
selection of one superior multicast routing protocol. It is not
impossible that different IP multicast routing protocols will be
deployed in the Internet.
3.1. Aggregation
In unicast different destination addresses are aggregated to one
entry in the routing table, yielding one FEC and one LSP.
The granularity of multicast streams is (*, G) for a shared tree and
(S, G) for a source tree, S being the source address and G the
multicast group address. Aggregation of multicast trees with
different multicast 'destination' addresses on one LSP is a subject
for further study.
3.2. Flood & Prune
To establish a multicast tree some IP multicast routing protocols
(e.g. DVMRP, PIM-DM) flood the network with multicast data. The
branches can then be pruned by nodes which do not want to receive the
data of the specific multicast group. This process is repeated
periodically.
Flood & Prune multicast routing protocols have some characteristics
which significantly differ from unicast routing protocols:
a) Volatile. Due to the Flood & Prune nature of the protocol, very
volatile tree structures are generated. Solutions to map a dynamic
L3 p2mp tree to a L2 p2mp LSP need to be efficient in terms of
signaling overhead and LSP setup time. The volatile L2 LSP will
consume a lot of labels throughout the network, which is a
disadvantage when label space is limited.
b) Traffic-driven. The router only creates state for a certain group
when data arrives for that group. Routers also independently decide
to remove state when an inactivity timer expires.
- Thus LSPs can not be pre-established as is usually done in
unicast. To minimize the time between traffic arrival and LSP
establishment a fast LSP setup method is favorable.
- Since creation and deletion of a L3 route at each node is
triggered by traffic, this suggests that the LSP associated with the
route be setup and torn down in a traffic-driven manner as well.
- If an LSR does not support L3 forwarding this traffic-driven
nature even requires that the upstream LSR takes the initiative to
create an LSP (Upstream Unsolicited or Downstream on Demand label
advertisement).
3.3. Source/Shared trees
IP multicast routing protocols create either source trees (S, G),
i.e. a tree per source (S) and per multicast group (G), or shared
trees (*, G), i.e. one tree per multicast group (Figure 1).
R1 R1 R1
S1 / / /
\ / / /
\ / / /
C---R2 S1---R2 S2---R2
/ \ \ \
/ \ \ \
S2 \ \ \
R3 R3 R3
Figure 1. Shared tree and Source trees
The advantage of using shared trees, when label switching is applied,
is that shared trees consume less labels than source trees (1 label
per group versus 1 label per source and per group).
However, mapping a shared tree end-to-end on L2 implies setting up
multipoint-to-multipoint (mp2mp) LSPs. The problem of implementing
mp2mp LSPs boils down to the merging problem discussed earlier.
Note that in practice shared trees are often only used to discover
new sources of the group and a switchover to a source tree is made at
very low bitrates.
3.4. Co-existence of Source and Shared Trees
Some protocols support both source and shared trees (e.g. PIM-SM) and
one router can maintain both (*, G) and (S, G) state for the same
group G. Two cases of state co-existence are described below.
Assume topologies with senders Si and receivers Ri. RP is the
Rendezvous Point. Ni are LSRs. The numbers are the interface
numbers, Reg is the Register interface. All IGMP and PIM Join/Prune
messages are shown in the figures. It is also indicated whether the
RPT-bit is set for the (S, G) state.
1) Figure 2 shows a switchover from shared to source tree. Assume
that the shortest path from R1 to RP is via N1-N2-N5. N1, the
Designated Router of receiver R1 (DRrecv), decides to initiate a
source tree for source S1. After the arrival of data via the source
tree in N2, N2 will send a prune to N5 for source S1. State co-
existence occurs in the node where the overlap of shared and source
tree starts (N2) and in the node where S1 does not need forwarding on
the shared tree anymore (N5).
PJ
IJ PJS PJS
-> 1 2 -> 1 2 -> 1 2
R1-----N1------N2------N3----S1
3| |3 IJ=Igmp Join
||PPS | PJ=Pim Join (*,G)
|vPJ | PJS=Pim Join (S1,G)
IJ PJ | PJ | PPS=Pim Prune (S1,G)
-> -> |3 -> |
R2-----N4------N5------RP----S2
1 2 1 2 1
Figure 2
The multicast routing states created in the MRT are:
in RP: (*,G):Reg->1 (i.e. incoming itf=Reg; outgoing itf=1)
in N1: (*,G):2->1
in N2: (*,G):3->1
(S1,G):2->1
in N3: (S1,G):2->Reg,1
in N4: (*,G):2->1
in N5: (*,G):2->1,3
(S1,G)RPT-bit:2->1
2) Figure 3 shows that even without a switchover, state co-existence
can occur. Multicast traffic from a sender will create (S, G) state
in the Designated Router of the sender (DRsend; N3 in Figure 3 is the
DRsend of S). Each node on a shared-tree has (*, G) state. Thus an
on-tree DRsend has both (*, G) and (S, G) state. If the DRsend is
on-tree it will also send a prune for S towards the RP, creating (S,
G) state in all nodes until the first router which has a branch (N1
and N2 in Figure 3).
S
PPS PPS |
PJ PJ PJ |2 PJ IJ
1 <- 1 3<- <- | <- <- PJ=Pim Join
RP------N1----N2----N3----N4----R1 IJ=Igmp Join
^|2 1 2 1 3 1 2 PPS=Pim Prune (S,G)
PJ|| IJ
1| <-
N5----R2
2
Figure 3
The multicast routing states created in the MRT are:
in RP: (*,G):Reg->1 (i.e. incoming itf=Reg; outgoing itf=1)
in N1: (*,G):1->2,3
(S,G)RPT-bit:1->2
in N2: (*,G):1->2
(S,G)RPT-bit:1->none
in N3: (*,G):1->3
(S,G):2->Reg,3
in N4: (*,G):1->2
in N5: (*,G):1->2
In the examples one can observe that two types of state co-existence
occur:
1) (S, G) with RPT-bit not set (N2 in Figure 2, N3 in Figure 3). The
(*, G) and (S, G) state have different incoming interfaces, but some
common outgoing interfaces. It is possible that the traffic of S
arrives on both the (*, G) and (S, G) interfaces. In normal L3
forwarding the (S, G)SPT-bit entry prohibits the forwarding of the
traffic from S arriving on the (*, G) incoming interface. The
traffic of S can only temporarily arrive on the incoming interfaces
of both the (*, G) and (S, G) entries (until N5 in Figure 2 and N1 in
Figure 3 have processed the prune messages). To avoid the temporary
forwarding of duplicate packets L3 forwarding can be applied in this
type of node. If one does not mind the temporary duplicate packets
L2 forwarding can be applied. In this case the (*, G) and (S, G)
streams have to be merged into the (*, G) LSP on their common
outgoing interfaces.
2) (S, G) with RPT-bit set (N5 in Figure 2, N1 in Figure 3). The (*,
G) and (S, G) state have the same incoming interface. The (S, G)
traffic must be extracted from the (*, G) stream. In MPLS this state
co-existence can be handled in several ways. Four approaches to this
problem will be described:
a) A first method to handle this state co-existence is to terminate
the LSPs and forward all traffic of this group at L3. However a
return to L3 can be avoided in case a (S, G) entry without outgoing
interface is added to the MRT (N2 in Figure 3). This entry will only
receive traffic temporarily. In this particular case one could
ignore the (S, G) state and maintain the existing (*, G) LSP, the
disadvantage being duplicate traffic for a very short time.
b) A second approach is to assign source specific labels on the nodes
of the shared tree. Multiple labels will be associated with one (*,
G) entry, corresponding to one label per active source. Since the
nodes only know which sources are active when traffic from these
sources arrives, the LSPs can not be pre-established and a fast LSP
setup method is favorable.
c) A third way is that only source trees are labelswitched and that
traffic on the shared tree is always forwarded at L3. This assumes
that the shared tree is only used as a way for the receivers to find
out who the sources are. By configuring a low bitrate switchover
threshold one can obtain that the receivers switchover to source
trees very quickly.
d) In the fourth approach an LSR which has (S, G)RPT-bit state with a
non-null oif, advertises a label for (S, G) to the upstream LSR and
this label advertisement is then propagated by each upstream LSR
towards the RP. In this way a dedicated LSP is created for (S, G)
traffic from the RP to the LSR with the (S, G)RPT-bit state. In the
latter LSR the (S, G) LSP is merged onto the (*, G) LSP for the
appropriate outgoing interfaces. This ensures that (S, G) packets
traveling on the shared tree do not make it past any LSR which has
pruned S.
3.5. Uni/Bi-directional Shared Trees
Bidirectional shared trees (e.g. CBT) have the disadvantage of
creating a lot of merging points (M) in the nodes (N) of the shared
tree. Figure 4 shows these merging points resulting from 2 senders S1
and S2 on a bidirectional tree.
S1 S2
|| ||
v| <- <- <- <- |v
<- <- | -> -> -> -> | ->
----N----M----M----M----M----M----N
|| || || || || ||
|v |v |v |v |v |v
| | | | | |
Figure 4. Multicast traffic flows from 2 senders on a bidirectional tree
In Figure 5 the same situation for unidirectional shared trees is
depicted. In this case the data of the senders is tunneled towards
the root node R, yielding only a single merging point, namely the
root of the shared tree itself.
S1
tunnel || S2
<----- v| tunnel ||
to R<------------------------- v|
-> -> | -> -> -> -> | ->
----N----N----N----N----N----N----N
|| || || || || ||
|v |v |v |v |v |v
| | | | | |
Figure 5. Multicast traffic flows from 2 senders on a unidirectional tree
3.6. Encapsulated multicast data
Sources of unidirectional shared trees and non-member sources of
bidirectional shared trees encapsulate the data towards the root
node. The data is then decapsulated in the root node. The
encapsulation and decapsulation of multicast data are L3 processes.
Thus in case of encapsulation/decapsulation a path can never be
mapped onto an end-to-end LSP: the traffic can not be forwarded on L2
on the Register interface of the DRsend (encapsulation), nor can it
cross the root (decapsulation) at L2.
Remarks:
1) If the LSR supports mixed L2/L3 forwarding (section 4), the (S, G)
traffic in DRsend can still be forwarded on L2 on the outgoing
interfaces which are not the Register interface.
2) The encapsulated traffic can also benefit from MPLS by label
switching the tunnels.
3) If the root node decides to join the source (to avoid
encapsulation/decapsulation), an end-to-end (S, G) LSP can be
constructed.
3.7. Loop-free-ness
Multicast routing protocols which depend on a unicast routing
protocol suffer from the same transient loops as the unicast
protocols do, however the effect of loops will be much worse in the
case of multicast. The reason being, each time a multicast packet
goes around a loop, copies of the packet may be emitted from the loop
if branches exist in the loop.
Currently loop detection is a configurable option in LDP and a
decision on the mechanism for loop prevention is postponed.
3.8. RPF Check
Some protocols perform a Reverse Path Forwarding (RPF) check on the
received multicast packets. This mechanism checks whether the packet
is received on the interface which is on the shortest path to the
source (or root). This mechanism can introduce problems when
explicit routing is used (see section 7). Indeed, explicit routing
can construct a tree yielding another incoming interface than the
RPF-compatible one.
3.9. Mapping of characteristics on existing protocols
The above characteristics are summarized in Table 1 for a non-
exhaustive list of existing IP multicast routing protocols: DVMRP
[PUSA], MOSPF [MOY], CBT [BALL], PIM-DM [DEER], PIM-SM [DEE2], SSM
[HOLB], SM [PERL].
+------------------+------+------+------+------+------+------+------+
| |DVMRP |MOSPF |CBT |PIM-DM|PIM-SM|SSM |SM |
+------------------+------+------+------+------+------+------+------+
|Aggregation |no |no |no |no |no |no |no |
+------------------+------+------+------+------+------+------+------+
|Flood & Prune |yes |no |no |yes |no |no |option|
+------------------+------+------+------+------+------+------+------+
|Tree Type |source|source|shared|source|both |source|shared|
+------------------+------+------+------+------+------+------+------+
|State Co-existence|no |no |no |no |yes |no |no |
+------------------+------+------+------+------+------+------+------+
|Uni/Bi-directional|N/A |N/A |bi |N/A |uni |uni |bi |
+------------------+------+------+------+------+------+------+------+
|Encapsulation |no |no |yes |no |yes |no |yes |
+------------------+------+------+------+------+------+------+------+
|Loop Free |no |no |no |no |no |no |no |
+------------------+------+------+------+------+------+------+------+
|RPF check |yes |yes |no |yes |yes |yes |no |
+------------------+------+------+------+------+------+------+------+
Table 1. Taxonomy of IP Multicast Routing Protocols
From Table 1 one can derive e.g. that DVMRP will consume a lot of
labels when the Flood & Prune L3 tree is mapped onto a L2 tree.
Furthermore since DVMRP uses source trees it experiences no merging
problem when label switching is applied. The table can be
interpreted in the same way for the other protocols.
4. Mixed L2/L3 forwarding in a single node
Since unicast traffic has one incoming and one outgoing interface the
traffic is either forwarded at L2 OR at L3 (Figure 6). Because
multicast traffic can be forwarded to more than one outgoing
interface one can consider the case that traffic to some branches is
forwarded on L2 and to other branches on L3 (Figure 7).
+--------+ +--------+
| L3 | | L3 |
| +>>+ | | |
| | | | | |
+--|--|--+ +--------+
| | | | | |
->-----+ +-----> ->------>>----->
| L2 | | L2 |
+--------+ +--------+
Figure 6. Unicast forwarding on resp. L3 or L2
+--------+ +--------+ +--------+
| L3 | | L3 | | L3 |
| +>>++ | | +>>+ | | |
| | || | | | | | | |
+--|--||-+ +--|--|--+ +--------+
| | |+----> | | +-----> | +---->
->-----+ | | | |L2 | ->----->>-+ |
| L2+-----> ->-----+>>------> | L2 +---->
+--------+ +--------+ +--------+
Figure 7. Multicast forwarding on resp. L3, mixed L2/L3 or L2
Nodes which support this 'mixed L2/L3 forwarding' feature allow that
a multicast tree splits in branches of which some are forwarded at L3
while others are switched at L2.
The L3 forwarding has to take care that the traffic is not forwarded
on those branches that already get their traffic on L2. This can be
accomplished by e.g. providing an extra bit in the Multicast Routing
Table.
Although the mixed L2/L3 forwarding requires processing of the
traffic at L3, the load on the L3 forwarding engine is generally less
than in a pure L3 node.
Supporting this 'mixed L2/L3 forwarding' feature has following
advantages:
a) Assume LSR A (Figure 8) is an MPLS edge node for the branch
towards LSR B and an MPLS rootcore node for the branch towards LSR C.
The mixed L2/L3 forwarding allows that the branch towards C is not
disturbed by a return to L3 in LSR A.
+-------------+
| MPLS cloud |
| N |
| / \ |
| / \ |
| / \ |
| A N |
|/ \ \ |
| \ \ |
/| \ |
B | C |
| |
+-------------+
Figure 8. Mixed L2/L3 forwarding in node A
b) Enables the use of the traffic driven trigger with the Downstream
Unsolicited or Upstream on Demand label distribution mode, as
explained in section 5.3.1.
5. Taxonomy of IP multicast LSP triggers
The creation of an LSP for multicast streams can be triggered by
different events, which can be mapped on the well known categories of
'request driven', 'topology driven' and 'traffic driven'.
a) Request driven: intercept the sending or receiving of control
messages (e.g. multicast routing messages, resource reservation
messages).
b) Topology driven: map the L3 tree, which is available in the
Multicast Routing Table, to a L2 tree. The mapping is done even if
there is no traffic.
c) Traffic driven: the L3 tree is mapped onto a L2 tree when data
arrives on the tree.
5.1. Request driven
5.1.1. General
The establishment of LSPs can be triggered by the interception of
outgoing (requiring that the label is requested by the downstream
LSR) or incoming (requiring that the label is requested by the
upstream LSR) control messages. Figure 9 illustrates these two
cases.
LSRu LSRd LSRu LSRd
-------+ +--- ---+ +-------
| control | | control |
<---*<-----message------- <-------message-------*----
| | | | | |
trigger| | | | | |trigger
| | bind | | bind | |
+--------or---------> <---------or----------+
| bind-request | | bind-request |
| | | |
| | | |
|----data----->| |-----data---->|
incoming outgoing
Figure 9. Request driven trigger
(interception of resp. incoming and outgoing control messages)
The downstream LSR (LSRd) sends a control message to the upstream LSR
(LSRu). In the case that incoming control messages are intercepted
and the MPLS module in LSRu decides to establish an LSP it will send
an LDP bind (Upstream Unsolicited mode) or an LDP bind request
(Downstream on Demand mode) to LSRd.
Currently, for multicast, we can identify two important types of
control messages: the multicast routing messages and the resource
reservation messages.
5.1.2. Multicast routing messages
In principle, this mechanism can only be used by IP multicast routing
protocols which use explicit signaling: e.g. the Join messages in
PIM-SM or CBT. Remark that DVMRP and PIM-DM can be adapted to
support this type of trigger [FARI], however, at the cost of
modifying the IP multicast routing protocol itself !
IP multicast routing messages can create both hard states (e.g. CBT
Join + CBT Join-Ack) and soft states (e.g. PIM-SM Joins are sent
periodically). The latter generates more control traffic for tree
maintenance and thus requires more processing in the MPLS module.
Triggers based on the multicast routing protocol messages have the
disadvantage that the 'routing calculations' performed by the
multicast routing daemon to determine the Multicast Routing Table are
repeated by the MPLS module. The former determines the tree that will
be used at L3, the latter calculates an identical tree to be used by
L2. Since the same task is performed twice, it is better to create
the multicast LSP on the basis of information extracted from the
Multicast Routing Table itself (see section 5.2 and 5.3). The
routing calculations become more complex for protocols which support
a switch-over from a (*, G) tree to a (S, G) tree because more
messages have to be interpreted.
When a host has a point-to-point connection to the first router one
could create 'LSPs up to the end-user' by intercepting not only the
multicast routing messages but the IGMP Join/Prune messages ([FENN])
as well.
5.1.3. Resource reservation messages
As is the case for unicast the RSVP Resv message can be used as a
trigger to establish LSPs. A source of a multicast group will send
an RSVP Path message down the tree, the receivers can then reply with
an RSVP Resv message. RSVP scales equally well for multicast as it
does for unicast because:
a) RSVP Resv messages can merge.
b) RSVP Resv messages are only sent up to the first branch which made
the required reservation.
5.2. Topology driven
The Multicast Routing Table (MRT) is maintained by the IP multicast
routing protocol daemon. The MPLS module maps this L3 tree topology
information to L2 p2mp LSPs.
The MPLS module can poll the MRT to extract the tree topologies.
Alternatively, the multicast daemon can be modified to notify the
MPLS module directly of any change to the MRT.
The disadvantage of this method is that labels are consumed even when
no traffic exists.
5.3. Traffic driven
5.3.1. General
A traffic driven trigger method will only construct LSPs for trees
which carry traffic. It consumes less labels than the topology
driven method, as labels are only allocated when there is traffic on
the multicast tree.
If the mixed L2/L3 forwarding capability (see section 4) is not
supported, the traffic driven trigger requires a label distribution
mode in which the label is requested by the LSRu (Downstream on
Demand or Upstream Unsolicited mode). In Figure 10, suppose an LSP
for a certain group exists to LSRd1 and another LSRd2 wants to join
the tree. In order for LSRd2 to initiate a trigger, it must already
receive the traffic from the tree. This can be either at L2 or at
L3. The former case is a chicken and egg problem. The latter case
requires a mixed L2/L3 forwarding capability in LSRu to add the L3
branch.
+--------+
| LSRd1 |
| |
+--------+ | L3 |
| LSRu | +--------+
| | | |
| L3 | +-------------------------->
+--------+ / | L2 |
| | / +--------+
->-------------+
| L2 | +--------+
+--------+ | LSRd2 |
| |
| L3 |
+--------+
| |
| |
| L2 |
+--------+
Figure 10. The LSRu has to request the label.
5.3.2. An implementation example
To illustrate that by choosing an appropriate trigger one can obtain
that MPLS multicast is independent of the deployed multicast routing
protocol following implementation example is given.
Current implementations on Unix platforms of IP multicast routing
protocols (DVMRP, PIM) have a Multicast Forwarding Cache (MFC). The
MFC is a cached copy of the Multicast Routing Table. The MFC
requests an entry for a certain multicast group when it experiences a
'cache miss' for an incoming multicast packet. The missing routing
information is provided by the multicast daemon. If at a later point
in time something changes to the route (outgoing interfaces added or
removed), the multicast daemon will update the MFC.
The MFC is implemented as a common component (part of the kernel),
which makes this trigger very attractive because it can be
transparently used for any IP multicast routing protocol.
Entries in the MFC are removed when no traffic is received for this
entry for a certain period of time. When label switching is applied
to a certain MFC-entry, the L3 will not see any packets arriving
anymore. To retain the normal MFC behavior, the L3 counters of the
MFC need to be updated by L2 measurements.
5.4. Combinations of triggers and label distribution modes
Table 2 shows the valid combinations of label distribution modes and
trigger types which were discussed in the previous sections. The (X)
means that the combination is valid when the mixed L2/L3 forwarding
feature is supported in the LSR.
+----------------+---------------------------------------------+
| | label requested by |
| | LSRu | LSRd |
| +----------------------+----------------------+
| | upstream |downstream|downstream |upstream |
| |unsolicited|on demand |unsolicited|on demand |
+----------------+-----------+----------+-----------+----------+
|Request Driven | | | | |
|(incoming msg) | X | X | | |
+----------------+-----------+----------+-----------+----------+
|Request Driven | | | | |
|(outgoing msg) | | | X | X |
+----------------+-----------+----------+-----------+----------+
|Topology Driven | X | X | X | X |
+----------------+-----------+----------+-----------+----------+
|Traffic Driven | X | X | (X) | (X) |
+----------------+-----------+----------+-----------+----------+
Table 2. Valid combinations of triggers and label distribution modes
6. Piggy-backing
In Figure 9 (outgoing case) one can observe that instead of sending 2
separate messages the label advertisement can be piggy-backed on the
existing control messages. For multicast two piggy-back candidates
exist:
a) Multicast routing messages: protocols as PIM-SM and CBT have
explicit Join messages which could carry the label mappings. This
approach is described in [FARI]. When different multicast routing
protocols are deployed, an extension to each of these protocols has
to be defined.
b) RSVP Resv messages: a label mapping extension object for RSVP,
also applicable to multicast, is proposed in [AWDU].
The pro and cons of piggy-backing on multicast routing messages will
be described now.
Piggy-backing has following advantages:
a) If the label advertisement is piggy-backed on multicast routing
messages, then the distribution of routes and the distribution of
labels is tightly synchronized. This eliminates difficult corner
cases such as "what do I do with a label if I don't (yet) have a
routing table entry to attach it to?". It also minimizes the
interval between the establishment of the multicast route and the
mapping of a label to the route.
b) The number of control messages needed to support label
advertisement beyond those needed to support the multicast routing
itself is zero.
Following disadvantages of piggy-backing can be identified:
a) In dense-mode protocols there are no messages on which the label
advertisement can be piggy-backed. [FARI] proposes to add periodic
messages to dense-mode protocols for the purpose of label
advertisement, which is a heavy impact on the multicast routing
protocol and it eliminates the message conserving benefit of piggy-
backing.
b) The second solution for the state co-existence problem (section
3.4) can not be applied in combination with piggy-backing.
c) Piggybacking requires extending the multicast routing protocol,
and hence becomes less attractive if label advertisement needs to be
supported for multiple multicast routing protocols. Especially when
not only the label advertisement but also the other two LDP functions
(discovery and adjacency) are piggy-backed.
d) Piggy-backing assumes the Downstream Unsolicited label
distribution mode, this excludes a number of trigger methods (see
Table 2).
e) LDP normally runs on top of TCP, assuring a reliable communication
between peer nodes. Piggy-backed label advertisement often replaces
the reliable communication with periodic soft-state label
advertisements. Because of this periodic label advertisement the
control traffic (in number of bytes) will increase.
f) If a VCID notification mechanism [NAGA] is required, the (in-band)
notification can normally be done by sending the LDP bind through the
newly established VC. This way only one message is required. This
method cannot be combined with piggy-backing because the routing
message is sent before the VC can be established. An extra handshake
message is thus required, diminishing the benefit of piggy-backing.
So whether piggy-backing makes sense or not depends heavily on which
and how many multicast routing protocols are deployed, whether LDP is
already used for unicast, which trigger mechanism is used, ... .
Piggybacking is just one possible component of an MPLS multicast
solution.
7. Explicit routing
Explicit routing for unicast refers to overriding the unicast routing
table by using LSPs.
A first way to interpret "multicast explicit routing" is overriding
the tree established by the multicast routing protocol by another LSP
tree (e.g. a centrally calculated Steiner tree).tree calculated by an offline tool). In this
interpretation the current 'shortest path' multicast routing protocol
becomes obsolete and can be replaced by label advertisement messages
that follow an explicit route which is e.g. calculated by an offline
tool.(e.g. a branch of the Steiner tree).
A second way of interpreting "multicast explicit routing" is that the
known multicast routing protocols useare running, but that the messages
generated by these protocols use explicit unicast routes (instead of
the IGP shortest path routes) to construct trees. This approach requires that the RPF check also
takes the explicit path into account.
8. QoS/CoS
8.1. DiffServ
The Differentiated Services approach can be applied to multicast as
well. It introduces finer stream granularities (DiffServ Codepoint
(DSCP) as an extra differentiator). A sender can construct one or
more trees with different DSCPs.
These (S, G, DSCP) or (*, G, DSCP) trees can be mapped very easily
onto LSPs when the traffic driven trigger is used. In this case one
can create LSPs with different attributes for the various DSCPs.
Note however that these LSPs still use the same route as long as the
tree construction mechanism itself does not take the DSCP as an
input.
8.2. IntServ and RSVP
RSVP can be used to setup multicast trees with QoS. An important
multicast issue is the problem of how to map the 'heterogeneous
receivers' paradigm onto L2 (remark that it is not solved in IP
either). This subject is tackled in [CRAW]. Pragmatic approaches
are the 'Limited Heterogeneity Model' which allows a best effort
service and a single alternate QoS (e.g. a QoS proposed by the sender
in a RSVP Path message) and the 'Homogeneous Model' which allows only
a single QoS.
The first approach will construct full trees for each service class.
The sender has to send its traffic twice across the network (e.g. 1
best-effort and 1 QoS tree). Both trees can be label switched.
The second approach constructs one tree and the best-effort users are
connected to the QoS tree. If the branches created for best-effort
users are not to be label switched, (thus carried by a hop-by-hop
default LSP) the QoS multicast traffic has to be merged onto these
default LSPs. This function can be provided by the 'mixed L2/L3
forwarding' feature described in section 4. If this is not available
merging is necessary to avoid a return to L3 in the QoS LSP.
The mapping of the IntServ service categories onto L2 for ATM service
categories is studied in [GARR].
9. Multi-access networks
Multicast MPLS on multi-access networks poses a special problem. An
LSR that wants to join a group must always be ready to accept the
label that is already assigned to the group LSP (to another
downstream LSR on the link). This can be achieved in three ways:
1) Each LSR on the multi-access link memorizes all the advertised
labels on the link, even if it has not received a join for the
associated group. If an LSR is added to the multi-access link it has
to retrieve this information from another LSR on the link or in case
of soft state label advertisement it can wait a certain time before
it can allocate labels itself. If LSRs allocate a label 'at the same
moment' the LSR with the highest IP address could keep it, while the
other LSRs withdraw the label.
2) Each LSR gets its own label range to allocate labels from. A
mechanism for label partitioning is described in [FAR2].[FARI]. If an LSR
is added to the multi-access link the label ranges have to be
negotiated again and possibly existing LSPs are teared down and are
reconstructed with other labels.
3) Per multi-access link one LSR could be elected to be responsible
for label allocation. When an LSR needs a label, it can request it
from this Label Allocation LSR.
Unlike the unicast case, a multicast stream can have more than one
downstream LSR which all have to use the same label. Two solutions
for label advertisement can be thought of:
1) [FARI] proposes to multicast the label advertisements to all LSRs
on the shared link. Since multicast is not reliable this requires
periodic label advertisements, yielding label advertisement
duplicates in time.
2) Another approach is that an LSR unicasts its label advertisements
in a reliable way (TCP) to all other (or to all interested) LSRs on
the shared link. In this approach the hard-state character of LDP
can be maintained but the label advertisement is duplicated in space.
Since LSPs are only rewarding if they have a long lifetime and since
the number of LSRs on a shared link is limited the second approach
seems advantageous.
Another issue with multicast in multi-access networks is whether to
use upstream or downstream label assignment. For multicast traffic,
upstream label allocation is simpler since there can be only one
upstream node per link that belongs to a multicast tree. This
(upstream) node can assign a label without any contention. With
downstream allocation, there may be multiple downstream nodes for a
given tree on a multi-access link; each node may propose a label
assignment leading to contention and a contention resolution scheme
is required to chose one of them as the label allocator.
Once a label has been assigned, it is possible that the label
assigner leaves the tree. With downstream label assignment, this
could happen when the label allocator leaves the group. With
upstream assignment this could happen when the upstream LSR changes
due to a unicast topology change.
10. More issues
10.1. TTL field
The TTL field in the IP header is typically used for loop detection.
In IP multicast it is also used to limit the scope of the multicast
packets by setting an appropriate TTL value.
Thus in LSRs that do not support a TTL decrement function (e.g ATM
LSR), the scope restriction function is affected. Suppose one could
calculate in advance the number of hops an LSP traverses. In a
unicast LSP the TTL value could then be decremented at the ingress or
the egress node. For multicast all the branches of the tree can have
different lengths so the TTL can only be decremented at the egress
node, potentially wasting bandwidth if the TTL turns out to be zero
or negative.
10.2. Independent vs. Ordered Label Distribution Control
Current Label Distribution Terminology is only defined for unicast.
The following sections explore what this terminology might mean in a
multicast context.
In Independent Control ([ANDE]) each LSR can take the initiative to
do a label mapping. In Ordered Control ([ANDE]) an LSR only maps a
label when it already received a label from its next-hop.
All the previously described trigger methods (section 5) combine with
Independent Control. Note that if the request driven approach is
used with Independent Control the label distribution still behaves as
in Ordered Control: the control messages flow from the egress node
upstream, imposing the same sequence to the label advertisement.
Ordered Control is not applicable for a traffic driven trigger in
case the node does not support mixed L2/L3 forwarding. According to
Table 2, this case implies that labels are requested by the upstream
LSR. Suppose in Figure 11 that an LSP exists from S to R1 and a new
branch must be added to R2. B will only accept a label on the A-B
link if a label is already assigned on the B-C link. However, to
establish a label on the B-C link, B must already receive traffic on
the A-B link.
N---N---R1
/
/
S -----A
\
\
B---C---R2
Figure 11.
10.3. Conservative vs. Liberal Label Retention Mode
In the Conservative Mode ([ANDE]) only the labels that are used for
forwarding data (if the next-hop for the FEC is the LSR which
advertised the label) are allocated and maintained. In the Liberal
Mode labels are advertised and maintained to all neighbors.
Liberal Mode does not make sense in multicast. Two reasons can be
identified for this:
1) All LSRs have a route for each unicast FEC. This is not true for
multicast FECs.
2) For multicast an LSR always knows to which neighbor to send the
label request or the label map messages. In e.g. unicast Downstream
Unsolicited mode (see below) the LSR does not know where to send the
label mappings and thus has to send the mapping to all its neighbors.
In this case supporting the Liberal Mode does not generate extra
messages (it still requires extra state information and label space)
and thus the threshold to support Liberal Mode could be considered
lower.
Table 3 shows the cases where it is known by an LSR where to send its
label requests.
+---------+----------------------------------+
| | label requested by |
| | LSRu | LSRd |
+---------+----------------+-----------------|
|unicast | Yes | No |
+---------+----------------+-----------------|
|multicast| Yes | Yes |
+---------+----------------+-----------------+
Table 3. Does an LSR know where to send its label requests ?
For a unicast flow, an LSR can determine the next hop LSR, which is
the one to send the request to in case of Upstream Unsolicited or
Downstream on Demand mode. The LSR is however not able to find the
previous hop. The previous hop is not necessarily the next hop
towards the source, because the path from A to B is not necessarily
the same as the path from B to A. Such a situation can occur as a
result of asymmetric link measures or in the event that multiple
equal cost paths exist [PAXS].
In the case of multicast, an LSR knows both the next hop(s) and the
previous hop. Because multicast trees are constructed using the
reverse shortest path method, the previous hop is always the next hop
towards the source or towards the root of the tree.
10.4. Downstream vs. Upstream Label Allocation
The label can be allocated by either the downstream LSR (Downstream
on Demand, Downstream Unsolicited) or the upstream LSR (Upstream on
Demand, Upstream Unsolicited, implicit). The advantages of
downstream label allocation are:
a) It is the same mode as for unicast LDP, thus eliminating the need
to develop upstream label distribution procedures.
b) The same label can be kept when the upstream LSR changes due to a
route change, which is an advantage on multi-access networks (see
section 9).
c) Compatible with piggybacking (especially the downstream
distribution mode).
The advantages of upstream label allocation are:
a) Easier label allocation in multi-access networks (see section 9).
b) The same label can be kept when the downstream LSR (which would
have been the label allocator in downstream mode in a multi-access
network) leaves the group (see section 9).
c) The upstream and implicit distribution mode allow a faster LSP
setup when the LSP is traffic triggered.
10.5. Explicit vs. Implicit Label Distribution
Beside the explicit distribution modes (which use a signaling
protocol), [ACHA] proposes an implicit label distribution method by
using unknown labels. This method has all the advantages of the
upstream label allocation method and is probably the fastest label
advertisement method for traffic triggered LSPs.
Implicit label distribution is not applicable if the FEC-to-label
binding has been advertised prior to traffic arrival, e.g. explicit
routing (i.e. if all the information necessary to identify the FEC is
not present in the packet).
Explicit distribution allows pre-establishment (before the arrival of
data) of LSPs with topology or request driven triggers.
11. Security Considerations
This document raises no security issues.
12. Acknowledgements
The authors would like to thank Eric Rosen, Piet Van Mieghem, Philip
Dumortier, Hans De Neve, Jan Vanhoutte, Alex Mondrus and Gerard
Gastaud for the fruitful discussions and/or their thorough revision
of this document.
References
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Cell Transport (IPSOFACTO) : Switching Multicast flows", IEEE
Globecom '97.
[AWDU] D. Awduche, L. Berger, D. Gan, T. Li, G. Swallow and V. Sriniva-
san, "Extensions to RSVP for LSP Tunnels", Work In Progress
[ANDE] L. Andersson, P. Doolan, N. Feldman, A. Fredette and R. Thomas,
"Label Distribution Protocol", Work In Progress."LDP specification", RFC3036, January 2001.
[BALL] A. Ballardie, "Core Based Trees (CBT) Multicast Routing Archi-
tecture", RFC2201, September 1997.
[CALL] R. Callon, P. Doolan, N. Feldman, A. Fredette, G. Swallow and A.
Viswanathan, "A Framework for Multiprotocol Label Switching",
Work In Progress.
[CONT] A. Conta, P. Doolan, A. Malis, "Use of Label Switching on Frame
Relay Networks", Work In Progress.RFC3034, January 2001.
[CRAW] E. Crawley, Editor, L. Berger, S. Berson, F. Baker, M. Borden
and J. Krawczyk, "A Framework for Integrated Services and RSVP
over ATM", RFC2382, August 1998.
[DAVI] B. Davie, J. Lawrence, K. McCloghrie, Y. Rekhter, E. Rosen, G.
Swallow and P. Doolan, "MPLS using LDP and ATM VC switching", Work In
Progress.RFC3035, January 2001.
[DEER] S. Deering, D. Estrin, D. Farinacci, A. Helmy, D. Thaler, S.
Deering, M. Handley, V. Jacobson, C. Liu, P. Sharma and L Wei,
"Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol
Specification", RFC 2117, June 1997.
[DEE2] S. Deering, D. Estrin, D. Farinacci, V. Jacobson, Protocol
Independent Multicast Version 2 Dense Mode Specification", Work
In Progress.
[FARI] D. Farinacci, Y. Rekhter and E. Rosen, "Using PIM to Distribute
MPLS Labels for Multicast Routes", Work In Progress.
[FAR2] D. Farinacci and Y. Rekhter, "Partitioning Label Space among
Multicast Routers on a Common Subnet", Work In Progress.
[FENN] W. Fenner, "Internet Group Management Protocol, IGMP, version
2", RFC 2236, November 1997.
[GARR] M. Garrett and M. Borden, "Interoperation of Controlled-Load
Service and Guaranteed Service with ATM", RFC2381, August 1998.
[HOLB] H. Holbrook, B. Cain, "Source-Specific Multicast for IP", Work
In Progress.
[MOY] J. Moy, "Multicast extensions to OSPF", RFC 1584, March 1994.
[NAGA] K. Nagami, N. Demizu, H. Esaki, Y. Katsube and P. Doolan, "VCID
Notification over ATM link for LDP", Work In Progress.RFC3038, January 2001.
[PERL] R. Perlman, C-Y. Lee, A. Ballardie, J. Crowcroft, Z. Wang, T.
Maufer, "Simple Multicast", Work In Progress.
[PUSA] T. Pusateri, "Distance Vector Multicast Routing Protocol", Work
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[ROSE] E. Rosen, Y. Rekhter, D. Tappan, D. Farinacci, G. Fedorkow, T.
Li, A. Conta, "MPLS Label Stack Encoding", Work In Progress.RFC3032, January
2001.
Authors Addresses
Dirk Ooms
Alcatel Corporate Research Center
Fr. Wellesplein 1, 2018 Antwerpen,Antwerp, Belgium.
Phone : 32 3 2404732
Fax : 32 3 2409879
E-mail: Dirk.Ooms@alcatel.be
Bernard Sales
Alcatel Corporate Research Center
Fr. Wellesplein 1, 2018 Antwerpen,Antwerp, Belgium.
Phone : 32 3 2409574
E-mail: Bernard.Sales@alcatel.be
Wim Livens
Colt Telecom
Zweefvliegtuigstraat 10, 1130 Brussel,Brussels, Belgium
Phone : 32 2 7901705
Fax : 32 2 7901711
E-mail: WLivens@colt-telecom.be
Arup Acharya
IBM TJ Watson Research Center
30 Saw Mill River Road, Hawthorne
NY 10532
Phone : 1 914 784 7481
E-mail: arup@us.ibm.com
Frederic Griffoul
Ulticom, Inc.
Les Algorithmes, 2000 Route des Lucioles, BP29
06901 Sophia-Antipolis, FRANCE
E-mail: griffoul@ulticom.com
Furquan Ansari
Bell Labs
101 Crawfords Corner Rd., Holmdel, NJ 07733
Phone : 1 732 949 5149
Fax : 1 732 332 6511
E-mail: furquan@dnrc.bell-labs.comyfurquan@dnrc.bell-labs.com